Various different gases may be supplied to a process chamber during the formation of a semiconductor or flat panel display device within the chamber. In a chemical vapour deposition process, gases are supplied to a process chamber housing the substrate and react to form a thin film over the surface of the substrate. For example, a LPCVD (low pressure chemical vapour deposition) nitride process uses DCS (dichlorosilane) and ammonia to form silicon nitride on the surface of a wafer. In an etch process, gases such as boron trichloride and chlorine may be supplied to the chamber to remove unwanted aluminium, and in a polysilicon etch process, hydrogen bromide and chlorine are supplied to the chamber. Cleaning gases such as the perfluorinated compounds CF4, C2F6, NF3 and SF6, and fluorine (F2) may be periodically supplied to the chamber to clean unwanted deposits from the chamber.
Some of these processes may lead to the formation of condensable species. The LPCVD process can produce HCl, which can react with excess ammonia to form aluminium chloride, and the metal etch process can produce aluminium chloride, each of which can readily condense within the foreline extending from the chamber outlet to a vacuum pump used to pump gas exhausted from the chamber, the exhaust line leading from the vacuum pump and/or within the vacuum pump itself. The polysilicon etch process can also lead to the formation of condensable species, in particular silicon tetrachloride. Condensation of particulates within the pump can result in the filling of the vacant running clearances between the rotor and stator elements of the pump, leading to a loss of pumping performance and ultimately pump failure. In view of this, it is common practice to supply an inert purge gas, typically nitrogen, at a flow rate of around 40 to 50 slm to the exhaust line or to one or more purge ports of the vacuum pump to reduce the partial pressure of the condensable species within the pump to a value at which substantially no condensation takes place within the pump.
Perfluorinated (PFC) cleaning gases are greenhouse gases, and so in view of this, before the exhaust gas is vented to the atmosphere, an abatement device is typically provided to treat the exhaust gas to convert the PFC gases into species that can be readily removed from the exhaust gas, for example by conventional scrubbing, and/or can be safely exhausted to the atmosphere. However, in view of the relatively high flow rate of purge gas added to the exhaust gas in comparison to the flow rate of the exhaust gas from the process chamber (typically around 5 slm), the addition of purge gas can significantly decrease the destruction efficiency of the abatement device. Abatement technology may involve contacting the exhaust gas with either reaction surfaces or high-energy discharges, and so the higher the mass flow rate through the abatement device, the lower the contact time between the exhaust gas and these surfaces or discharges, and hence the lower the destruction efficiency. This is particularly important in relation to the perfluorinated cleaning gases such as CF4 and SF6, which tend to be relatively difficult to destroy.